diff --git a/promql/lex.go b/promql/lex.go
index 5d390ca11e..743a528d6e 100644
--- a/promql/lex.go
+++ b/promql/lex.go
@@ -341,7 +341,7 @@ func (l *lexer) ignore() {
 
 // accept consumes the next rune if it's from the valid set.
 func (l *lexer) accept(valid string) bool {
-	if strings.IndexRune(valid, l.next()) >= 0 {
+	if strings.ContainsRune(valid, l.next()) {
 		return true
 	}
 	l.backup()
@@ -350,7 +350,7 @@ func (l *lexer) accept(valid string) bool {
 
 // acceptRun consumes a run of runes from the valid set.
 func (l *lexer) acceptRun(valid string) {
-	for strings.IndexRune(valid, l.next()) >= 0 {
+	for strings.ContainsRune(valid, l.next()) {
 		// consume
 	}
 	l.backup()